Remove unnecessary logic from ArgSplitter #25008

Job	Run time
Classify changes	5s
Bootstrap Pants, test Rust (macOS13-x86_64)	3m 37s
Bootstrap Pants, test and lint Rust (Linux-x86_64)	2m 8s
Bootstrap Pants, test Rust (Linux-ARM64)	1m 49s
Build wheels (Linux-ARM64)	0s
Ensure PR has release notes	3s
Build wheels (Linux-x86_64)	0s
Build wheels (macOS13-x86_64)	0s
Build wheels (macOS14-ARM64)	0s
Test Python (Linux-x86_64) Shard 1/10	18m 15s
Lint Python and Shell	3m 31s
Test Python (Linux-x86_64) Shard 0/10	18m 32s
Test Python (Linux-x86_64) Shard 2/10	18m 29s
Test Python (Linux-x86_64) Shard 3/10	16m 14s
Test Python (Linux-x86_64) Shard 4/10	31m 33s
Test Python (Linux-x86_64) Shard 5/10	14m 46s
Test Python (Linux-x86_64) Shard 6/10	10m 27s
Test Python (Linux-x86_64) Shard 7/10	16m 55s
Test Python (Linux-x86_64) Shard 8/10	17m 17s
Test Python (Linux-x86_64) Shard 9/10	10m 48s
Test Python (Linux-ARM64)	9m 51s
Test Python (macOS13-x86_64)	28m 54s
Set Merge OK	1s
Merge OK	1s
	3h 43m 16s

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